(a) Field of the Invention
The field of this invention relates to airfoils in the form of flexible wing sails used to generate propelling forces.
(b) Description of the Prior Art
The evolutionary course of sail design has long been in the pursuit of utilizing wing forms featuring two fairing surfaces forming aerodynamically efficient shapes with a draft area between them at their typical chordal plane.
Briefly, the fundamental aerodynamic advantage inferred in the usage of a wing form as a sail, and henceforth an airfoil, is in the spatial relationship around a chordal draft area and between the two separate surfaces with cambers designed to provide different flow lengths after the separation line along the luff, hence in their capability of basically providing a higher surface pressure differential between the windward and the leeward surfaces as opposed to single ply conventional sails and is, in itself, a well established phenomenon as taught by the discipline of aerodynamics.
In pursuit of achieving maximum possible performance, prior art in sail design has numerous examples of attempting to create wing shaped sails both in theory and in practice, which can be observed in general categories such as:
A--Wing structures with symmetrical chordal profiles rotatable around a vertical axis, PA0 B--Composite chordal profiles such as: PA0 C--Asymmetrical chordal profiles:
(1) wing masts with symmetrical chordal profiles also rotatable around a vertical axis, carrying single ply sails, PA1 (2) plurality of rigid wing structures with symmetrical chordal profiles movably related to each other to create a general asymmetrical chordal profile and sail twist in the totality of such assemblies, PA1 (1) two ply sails wrapped around a spar, providing a relatively less turbulent separation zone, as well as a better flow length ratio between the windward and the leeward plies at both tacks, PA1 (2) wings with asymmetrical but fixed chordal profiles that can only be tacked when they are rotated end to end around a pivot at the center of their spans and around a horizontal axis, PA1 (3) plurality of wing structures with asymmetrical but rigid chordal profiles attached to support structures that can be rotated around a horizontal axis so that the totality of the wing assembly can be tacked, PA1 (4) padded sails consisting of two plies with shaped foam layers placed in between, PA1 (5) double luffed sails.
(c) The objective of the invention
In view of the past pursuits of the prior art in wing sail design, the relevant data as shown by aerodynamics, and the entire scope of the operational conditions encompassing the full spectrum of sail use, the functional and operational criteria which constitute the optimum characteristics of a sail assembly can be summarized as follows:
(1) A form that can provide aerodynamic efficiency as close as possible to the theoretical limits of performance at all attitudes and conditions of operation
(2) The capability of being hoisted, tacked, trimmed, reefed, lowered and stowed away
(3) The capability of performing the functions listed above without applying any external control forces other than conventional tension control means generally used in ordinary sail assemblies.
(4) The capability of being luffed with efficiency such as in the fluttering action of single ply conventional sails,
(5) Inherent simplicity of structural, mechanical and material design to the end of requiring relative simplicity in manufacturing, maintenance and repair also achieving cost efficiency.
The existing solutions to the entirety of the problems outlined above do not include any variations that at once meet all of the mechanical, operational and aerodynamic functionality requirements generally suggested by the criteria mentioned in the preceding paragraphs. The objective of this invention is to create a wing sail assembly with novel mechanical characteristics to meet all the requirements of the functional and operational criteria mentioned hereto.